In U.S. Pat. No. 3,626,257, (the Esaki patent) in which two of the inventors hereof, Leo Esaki and Raphael Tsu, are coinventors, there is disclosed a new class of devices which exhibited a DC negative resistance. The new class of devices were referred to as semiconductor devices with a superlattice region. The aforesaid patent taught that by producing a device having a portion (a superlattic region) which exhibits a periodic potential difference from that of a uniform crystal lattice for the carriers of the material to interact with, a high-frequency bulk negative resistance device could be achieved.
The Esaki patent taught that a superlattice region could be produced by either doping or alloying techniques. The alloying referred to in the Esaki patent contemplated a first layer of a first material or alloy and a second layer which was an alloy which had as a constituent part thereof, the original material or alloy. From this type of alloying procedure, a periodic electron energy vs. distance characteristic was developed as shown in FIG. 3 of the Esaki patent. It will be noted that alloying of the materials specified in the Esaki patent results in the adjustment of the gap energy between alternate layers such that if the conduction band is raised, the valence band is lowered. The alloy type superlattice structures disclosed in the Esaki patent relied upon interactions between the conduction bands to produce a negative resistance.
The alloy type superlattice structures disclosed in the Esaki patent required higher current levels (higher fields) before the negative resistance region is reached. Thus, large DC bias power is required to obtain the negative resistance characteristic. As a result, the ratio of signal power to bias power is low.
As mentioned above, the Esaki patent also discusses the construction of superlattice regions employing alternately doped layers, the energy level diagrams of which are shown in FIG. 2 of the Esaki patent. It is now believed that practical superlattice structures cannot be effectively constructed by doping techniques since in order to produce the energy vs. distance characteristic desired, such heavy doping is required that it reduces mobility of the carriers to a point that the interactions contemplated may not occur.